This invention relates to methods and apparatus for forward error correction coding, and more particularly to such methods and apparatus for use in digital audio broadcasting systems.
Digital Audio Broadcasting (DAB) is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. AM In-Band, On-Channel (IBOC) DAB can be transmitted in a hybrid format where a digitally modulated signal coexists with the AM signal, or it can be transmitted in an all-digital format where the removal of the analog signal enables improved digital coverage with reduced interference. The hybrid format allows existing receivers to continue to receive the AM signal while allowing new IBOC receivers to decode the DAB signal. In the future, when IBOC receivers are abundant, a broadcaster may elect to transmit the all-digital format. The DAB signal of the all-digital format is even more robust than the hybrid DAB signal because of allowed increased power of the former with a digital time diversity backup channel. IBOC requires no new spectral allocations because each DAB signal is simultaneously transmitted within the spectral mask of an existing AM channel allocation. IBOC promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners.
U.S. Pat. No. 5,588,022 teaches a method for simutlanusly broadcasting analog and digital signals in a standard AM broadcasting channel. An amplitude modulated radio frequency signal having a first frequency spectrum is broadcast. The amplitude modulated radio frequency signal includes a first carrier modulated by an analog program signal. Simultaneously, a plurality of digitally modulated carrier signals are broadcast within a bandwidth that encompasses the first frequency spectrum. Each of the digitally modulated carrier signals is modulated by a portion of a digital program signal. A first group of the digitally modulated carrier signals lies within the first frequency spectrum and is modulated in quadrature with the first carrier signal. Second and third groups of the digitally modulated carrier signals lie outside of the first frequency spectrum and are modulated both in-phase and in-quadrature with the first carrier signal. U.S. patent application Ser. No. 09/049,217, assigned to the same assignee as the present invention, discloses another embodiment of an AM Digital Audio Broadcasting system.
Coding for an IBOC DAB system has been described in: B. Kroeger, D. Cammarata, xe2x80x9cRobust Modem and Coding Techniques for FM Hybrid IBOC DAB,xe2x80x9d IEEE Trans. on Broadcasting, Vol. 43, No. 4, pp. 412-420, December 1997. Trellis coded modulation has been proposed for use in AM IBOC DAB systems. Pragmatic trellis coded modulation (PCTM) has been described in: A. Viterbi, et al., xe2x80x9cA Pragmatic Approach to Trellis-Coded Modulation,xe2x80x9d IEEE Communications Magazine, pp. 11-19, July 1989. The use of complementary punctured codes has also been proposed for IBOC DAB systems. Complementary punctured codes have been described in: S. Kallel, xe2x80x9cComplementary Punctured Convolution (CPC) Codes and Their Applications,xe2x80x9d IEEE Trans. Comm., Vol 43, No. 6, pp. 2005-2009, June 1995. The present invention seeks to provide an improved forward error correction method for use in AM IBOC DAB transmitters. Receivers that process signals that were transmitted in accordance with the method are also described.
A method for digital audio broadcasting comprises the steps of providing a plurality of bits of digital information to be transmitted, interleaving the bits of digital information into a plurality of partitions, forward error correcting the bits of digital information using a combination of pragmatic trellis code modulation and complementary punctured code, and transmitting the bits of digital information. The partitions include a main partition, a backup partition, an upper partition, and a lower partition, wherein the upper partition and lower partition are non-overlapping. The main partition overlaps both of the upper and lower partitions and the backup partition overlaps both of the upper and lower partitions. The interleaved bits comprise a core interleaver and an enhancement interleaver. The forward error correcting is accomplished by representing the bits as independently coded in-phase and quadrature signals, applying a first error correcting code to the in-phase signals, and applying a second error correcting code to the quadrature signals. Receivers for processing the broadcast signals are also provided.